1. Field of the Invention
The present invention relates to a resist coating method and a resist coating apparatus which apply or coat a resist solution to a substrate to be processed like a semiconductor wafer.
2. Description of the Related Art
A photolithographic process step in a semiconductor device manufacturing process includes, for example, a resist coating step for forming a resist film on a semiconductor wafer and a developing step for developing a resist-coated wafer after an exposure process is performed on the resist-coated wafer. A spin coating method is widely used in the resist coating step to coat a resist solution uniformly over the wafer surface.
According to the spin coating method, a semiconductor wafer is rotated together with a spin chuck by rotational drive means with the semiconductor wafer vacuum-fixed on the spin chuck, and a resist nozzle positioned above the semiconductor wafer drops a resist solution onto the center of the wafer surface. The dropped resist solution spreads radially outward toward the circumference of the semiconductor wafer by centrifugal force. Although the dropping of the resist solution is terminated thereafter, the semiconductor wafer is continuously rotated while decreasing the rotational speed to spin off and dry the resist solution spread over the wafer surface.
Recently, reduction of resist consumption, i.e., reduction of the amount of the resist solution dropped on a wafer is strongly desired from the viewpoint of reducing the manufacturing cost or the like. When the amount of the resist solution dropped on a wafer is relatively large, the thickness of a coated film is easily adjustable to ensure a uniform thickness. When the amount of the resist solution dropped on the wafer is reduced, however, even if the conventional spin coating method can coat the resist solution to the entire wafer surface, the adjustment of the thickness of a coated film is difficult, making it difficult to achieve the uniform thickness. Specifically, with a small amount of the resist solution dropped on the wafer, as drying the resist solution rapidly progresses before the dropped resist solution sufficiently spreads toward the circumference of the wafer, raising a problem on the distribution of the thicknesses of the coated film such that the thickness of the outer circumference portion of the wafer is thinner than that of the central portion thereof. Accordingly, adjustment of the thickness is difficult, making it difficult for the coated film to have a uniform predetermined thickness.
As a method of adjusting the thickness of a coated film uniformly even with a reduced amount of a resist solution dropped, Japanese Patent Laid-open Publication No. H11-260717 discloses a resist coating method that includes a step of applying (coating) a resist solution to a target substrate to be processed while spreading the resist solution radially outward toward the circumference of the target substrate by supplying the resist solution to the approximate center of the target substrate while rotating the target substrate at a first rotational speed, a step of adjusting the thickness of the coated film by reducing the rotational speed of the target substrate to a second rotational speed lower than the first rotational speed after supply of the resist film is terminated, and a step of spinning off a residue of the resist solution by accelerating the rotational speed of the target substrate to a third rotational speed higher than the second rotational speed. The method disclosed in the publication is excellent in that the thickness of the resist film can be made uniform over the entire target substrate even if the supply amount of the resist solution is significantly reduced. Recently, reduction in the supply amount of the resist solution is demanded more than ever, and there is a demand to reduce the supply amount of the resist solution by even a tiny amount of 1/10 ml. However, because the method disclosed in Japanese Patent Laid-open Publication No. H11-260717 rapidly decelerates the rotation of the target substrate from the first rotational speed to the second rotational speed, a deceleration shock causes the resist solution that is spreading around the wafer surface to return from the outer circumference of the wafer toward the center thereof. This increases the supply amount of the resist solution even slightly (for example, 0.1 ml or so). That is, the method disclosed in the publication allows the excess resist solution to realize the uniformity of the thickness of the coated film even if the amount of the excess resist is minimal. From the standpoint of reducing the supply amount of the resist solution, there still is a room for improvement.